A holographic recording system has been known in the art, which employs holograms to record information in an optical disc recording medium at an ultra-high density. In the holographic recording system, an information light for carrying image information is superimposed onto a recording reference light in a recording layer inside the optical disc recording medium to generate an interfered fringe pattern. The fringe pattern is recorded in the recording layer of the optical disc recording medium to write the image information. When information is regenerated from the recorded fringe pattern, a regenerating reference light similar to that on writing is irradiated to the fringe pattern recorded in the optical disc recording medium to cause diffraction by the fringe pattern for regeneration of image information.
Recently, attention is given to the development of volume holography, which also utilizes the thickness of the recording layer in the optical disc recording medium to write fringe patterns three-dimensionally for achievement of a further increased recording density. A recording system according to the volume holography can be employed for further multiplex recording to increase the recording capacity dramatically.
An apparatus and method according to the volume holography for recording and regenerating information to/from an optical disc recording medium is disclosed in an international publication of WO 99/44195. In order to provide an understanding of the present invention, a brief description is given to the optical disc recording medium for recording the volume holography disclosed in the publication. As shown in FIG. 13, an optical disc recording medium 101 comprises a recording layer 101c for hologram recording provided between circular transparent substrates 101a, 101b, and a reflective film 101d formed on a surface of the transparent substrate 101b opposite to the recording layer 101c. They are bonded to a substrate 101e. The reflective film 101d has a plurality of address servo areas arranged at a certain angular interval in a radial direction of the optical disc recording medium 101. An information recording area is provided between the address servo areas arranged in a circumferential direction. Servo information for focusing serve control and tracking servo control and address information about the information recording area are previously recorded (pre-formatted) with embossed pits 101fin the address servo area.
In a specific configuration of the optical disc recording medium, the transparent substrates 101a, 101b have thicknesses of about 500 μm, and the recording layer 101c has a thickness of about 200 μm. The recording layer 101c is composed of a hologram recording material that varies optical properties such as an index of refraction, a permittivity and a reflectance in accordance with the intensity of a laser light when it is exposed to the laser light for a certain period of time.
In an example of recording by volume holography to the recording layer 101c, an information light 111 carrying information to be recorded and a recording reference light 112 together produce an interfered fringe pattern along the thickness in the recording layer 101cas shown. The information light 111 and the reference light 112 are so irradiated simultaneously for a certain period of time from beyond the transparent substrate 110ato fix the fringe pattern three-dimensionally in the recording layer 101c. The information is thus recorded as a three-dimensional hologram.
The information to be recorded in the recording layer 101ccomprises an information pattern arranged two-dimensionally as shown in FIG. 14, which is given from a spatial optical modulator 113. The information pattern can be obtained by controlling transmission/non-transmission of light through each of two-dimensionally arranged dots 114 that configure the spatial optical modulator. In this case, a spike-like light intensity distribution appears on the hologram surface at a period determined by an arranging pitch pD of the dots 114 and a focal length f of the lens and a wavelength λ of the recording light and prevents linear recording. Therefore, a defocusing method is employed in the art to slightly displace the hologram surface from the focal length to balance the light intensity distribution. Particularly, in the recording medium with the embossed pits 101fformed therein, location of the recording layer 101c too close to the embossed pit 101fcauses an inconvenience on hologram recording/regenerating. Therefore, a transparent substrate 101bwith a certain thickness is interposed between the recording layer 101c and the reflective film 101d. 
If the thickness of the transparent substrate 101bis too thick, however, a hologram to be recorded has an excessively larger diameter D, which lowers the recording density. An increase in recording density results in an overlap between adjacent holograms. A numeric value indicative of the number of possibly multiplexed holograms that can achieve 100% diffraction efficiency is called M number. The M number is determined by the recording material. If the multiplex number of holograms is equal to M, the regeneration efficiency η is proportional to the square of a value derived from division of the M number by the multiplex number M. Therefore, elevation of the regeneration efficiency requires the multiplex number M to be reduced as low as possible. This requires the transparent substrate 101b to have an appropriate thickness.
The present invention has been made in consideration of such the matters and has an object to provide an optical disc recording medium capable of achieving a high regeneration efficiency and recording at a high recording density and method of manufacturing the same.